All unit tests throwing BadImageFormatException with Moq?

I'm currently in the process of increasing code coverage on our software products and have ran into an issue; all of my unit tests (when compiled using 'Any CPU') are failing due to throwing a 'BadImageFormatException'.

This exception can be circumvented by building the solution using 'x86' instead of 'Any CPU', however the requirements are such that we need to be able to run them using Any CPU/x64 bit.

All unit tests involving Moq follow pretty much the same format:

[TestMethod]
public void GetProduct_ValidId_ProductReturned()
{
    //Setting up the object
    Product prod = new Product();
    prod.ID = 7;
    prod.Name = "Test";

    //Create the mocks
    var mockProductRepo = new Mock<IRepository<Product>>();
    var testDb = new Mock<IUnitOfWork>();

    //Setup what the repo needs to return, in this case it's a Product
    mockProductRepo.Setup(m => m.getByID(7)).Returns(prod); 

    //Setup what the database needs to return, in this case it's our repo which we've already setup
    testDb.SetupGet(m => m.ProductRepo).Returns(mockProductRepo.Object);

    //Run the test
    Product returnedProd = ProductHelper.getProduct(testDb.Object, 7);
    Assert.IsNotNull(returnedProd);
}

I can confirm that this test is successful when it is built using x86. Does anyone have any ideas on how I can get Moq to play nice when built using 'Any CPU'?

As an aside I can also confirm that all my projects in the solution are build using the same value ('Any CPU'). I'm using Moq v4.0.